1. Field of the Invention
This invention relates to broadband communication systems, and more particularly those that use the Data Over Cable Service Interface Specification (DOCSIS) media access protocol or its derivatives.
2. Description of the Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems, to the Internet, to cable systems, to local area networks (LANs), to wide area networks (WANs) to in-home wireless networks and the like. Often, these systems are comprised of numerous different forms of transmission media.
In the example of two-way data communications via satellite, data may be transmitted using time division multiplexing (TDM) over a single carrier (i.e. channel). A gateway receives data from a network such as the Internet, performs forward error correction (FEC) then modulates the data. The data is transmitted up to a satellite and back down from the satellite to one or more receivers.
For each communication device to participate in communications of whatever type, it includes or is coupled to a transceiver (i.e., for terrestrial wireless, a radio receiver and transmitter; and for two-way satellite, a satellite receiver and transmitter). As is known, the transmitter of radio and satellite transceivers include a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless or satellite communication standard. The one or more intermediate frequency stages mix the baseband signals with the signal generated by one or more local oscillators to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna or satellite dish.
As is also known, the receiver of a transceiver is also coupled to the antenna or satellite dish and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery (i.e. demodulation) stage. The low noise amplifier receives an inbound RF signal via the antenna or satellite dish and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with the signal generated by one or more local oscillators to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. This is typically referred to as frequency down-conversion. The filtering stage filters the down-converted baseband or IF signal to eliminate unwanted out of band signals to produce a filtered signal that is only that which falls within the bandwidth of the selected channel. Thus, this filter is sometimes referred to as a channel select filter. The data recovery or demodulation stage recovers raw data from the filtered signal in accordance with the particular wireless or satellite communication standard.
One common data network architecture, specified as the Data Over Cable Service Interface Specification) DOCSIS1, originated with cable operators interested in deploying high-speed packet-based communications systems on cable television systems. These include IP based Internet data, packet telephony service, video conferencing service, and many others. The goal of DOCSIS is to define a data service that will allow transparent bi-directional transfer of Internet Protocol (IP) traffic between a cable system headend or Cable Modem Termination System (CMTS) and customer locations using a cable modem (CM), over an all-coaxial or hybrid-fiber/coax (HFC) cable network. 1The DOCSIS Radio Frequency Interface Specification SP-RFIv1.1-I09-020830 is publicly available from and is hereby incorporated herein by this reference for all purposes.
The DOCSIS Media Access Control (MAC) sublayer specifies that the CMTS provide a single carrier transmitter for each downstream (i.e. from head-end to subscriber) channel. All CMs at subscriber locations listen to all frames transmitted on the downstream channel upon which they are registered and accept those frames where the destinations match the CM itself or CPEs (customer premises equipment). CMs can communicate with other CMs only through the CMTS.
The upstream channel is thus characterized by many transmitters (CMs) and one receiver (the CMTS). Time in the upstream channel is slotted, providing for Time Division Multiple Access at regulated time ticks. The CMTS provides the time reference and controls the allowed usage for each interval. Intervals may be granted for transmissions by particular CMs, or for contention by all CMs. CMs may contend to request transmission time. To a limited extent, CMs may also contend to transmit actual data. In both cases, collisions can occur and retries are then used.
The DOCSIS protocol has been adapted to other types of media, including terrestrial fixed wireless and two way satellite. For these applications, as well as the original data over cable service, data is transferred between a central location and many remote subscribers. The term for the centrally located equipment for broadband terrestrial fixed wireless systems is a Wireless Access Termination System (WATS). The subscriber equipment is called a wireless modem. With respect to two way satellite, the centrally located equipment is a satellite gateway (SG), while the subscriber equipment is a satellite modem (SM). Those of average skill in the art will recognize that in each of these types of service, the DOCSIS architecture is substantially maintained, even if some of the implementation details are adapted to the type of media used for transmission.
In standard DOCSIS based systems such as those described above, the downstream transmission is defined to be a time division multiplexed (TDM) signal with a fixed modulation type as well as a fixed forward error correction (FEC) coding rate. Thus, by nature the downstream signal has a fixed spectral efficiency in bits per second/Hertz [bps/Hz]. Signal parameters such as the modulation type, FEC coding type, and FEC coding rate determine the minimum signal to noise ratio (SNR) that must be present for the SM to have error-free or quasi error-free operation in a given channel having those parametric limitations. Thus, there is an inherent trade-off between the values of receiver parameters that yield a high level of throughput (e.g. high-order modulation and high FEC code rates) and those values (e.g. low-order modulation and more robust but lower FEC code rates) that ensure that the signal can be reliably received under conditions of low SNR but with lower throughput.
In many real world environments, subscribers of such systems experience a wide range of path losses and channel degradations. One example is a satellite based system where a downstream spot beam broadcasts to SMs that are located over a wide geographic area. Various conditions such as localized rainfall, partial obstructions, antenna misalignments, etc. can significantly degrade the signal power levels (and thus SNRs) received by individual subscribers. Those of average skill in the art will recognize that similar channel degradation may be experienced for subscribers of terrestrial fixed wireless and even data over cable, although the causes may be different.
FIG. 1 illustrates the basic elements of a two-way satellite system. A satellite gateway (SG) with a baseband modulator/demodulator 100 receives data from a network, such as the Internet. The data is assembled into an appropriate format in accordance with, for example the DOCSIS architecture previously described, and is then provided to transceiver 102. The transceiver performs certain functions necessary for transmitting the data using the satellite dish 104, up to the satellite 106 and down to a plurality of SMs 112 over downstream channel 114. The downstream signal is received by the dish 108, processed by the transceiver 110, and demodulated by SM 112. The SMs 112 transmit data, generated by the customer premise equipment (CPE) (not shown) back to the SG 100 over the upstream channel 116 uses the format recognized by the SG 100.
FIG. 2 is a block diagram illustrating the processing blocks of a known SG 100a, along with the processing blocks of the transceiver 102. Data from a network 250, such as the Internet, is transmitted between the network and the gateway DOCSIS MAC 204a. The data is formatted in accordance with the DOCSIS protocol. This protocol uses an MPEG format in the “downstream transmission convergence sublayer” that serves as the interface between the MAC and physical layer (PHY). MPEG specifications are publicly available and are incorporated herein by this reference for all purposes.
The downstream MPEG data stream that is output at 240a from the MAC 204a is encoded and modulated using a single type of modulation and a single set of FEC parameters by the fixed encoding and modulation stage 206a. The modulated and encoded MPEG stream is then up converted and filtered, and then fed into a high power amplifier by transceiver 214. This signal is transmitted continuously in a single frequency band, through a satellite (106, FIG. 1) and received by subscriber SMs (112, FIG. 2).
The SG 100a also receives (i.e. at antenna dish 104, FIG. 1) the upstream signal transmitted by subscriber satellite modems SM 112 of the system. The signal is filtered and down converted back to baseband at stage 212 using a fixed set of demodulation and decoding parameters at stage 208a to recover the upstream data stream.
FIG. 3 is a block diagram illustrating the processing blocks of a known SM 112a, along with the processing blocks of the transceiver 110. The SM 112a receives the signal through dish 108 and down converts the signal using a fixed set of demodulation and decoding parameters to recover the MPEG stream at processing box 540. The information is processed by the local DOCSIS MAC 504a in conjunction with the host processor 500a. The data is then passed on to the CPE of the subscriber at line 250.
Note that the modulation type and the FEC encoding parameters are fixed for all data transmitted by the SG 100a and received by each SM 112a over the downstream channel of the system of FIGS. 2 and 3. Indeed, to ensure that customers do not experience total loss of service under conditions of low SNR, current DOCSIS based systems must operate with channel parameters (and therefore fixed modulation and FEC encoding parameters) that ensure that even the subscriber situated the worst in terms of signal degradation (as manifested by bit error rate or SNR) is able to obtain service with a high probability of success. As a result, the majority of subscribers that could otherwise receive data at a higher rate are penalized by the presence of the relatively fewer environmentally disadvantaged subscribers.